1. Field
An aspect of the present invention relates to a camera head separated type camera device which has a camera head separated from a camera control unit for controlling the camera head.
2. Description of the Related Art
There is know a camera head separated type camera in which a camera head having an image pickup element such as a CMOS is detachably attached to a camera control unit (hereinafter referred to as CCU) for controlling the operation of the camera head through a camera cable. For example, such camera is used as an endoscope for medical or industrial use.
In such camera device, a video signal obtained from the image pickup element in the camera head is fed to the CCU through a dedicated cable. In recent years, the video signal is digital-transmitted as an LVDS (Low Voltage Differential Signaling) signal with advance of high resolution and high speed. It is said that LVDS enables signal processing with high speed transmission, small signal amplitude, low power consumption and little electromagnetic interference.
When the transmission distance of the digital video signal from the camera head to the CCU becomes long, it is difficult to transmit a digital video signal correctly. In view of this difficulty, there has been proposed a technique in which frequency characteristic of a digital video signal to be transmitted to a transmission target via a prearranged transmission line is corrected in advance so that degradation of the frequency characteristic of the digital video signal via the transmission line can be compensated based on the signal level of a reference signal transmitted via the transmission line (e.g. see JP-H08-317251-A).
When the cable length is long at the time of transmission of a LVDS video signal from the camera head to the CCU, the amplitude of the differential signal is lowered because of a voltage drop due to the internal resistance of the cable, and the rising time and the falling time of the differential signal are elongated because of the frequency characteristic of the video signal. As a result, a data reproducing condition of the CCU may not be satisfied so that accurate information transmission cannot be performed.
FIG. 13 illustrates the example waveform of an LVDS signal. When pulses of the LVDS signal continue with a constant rising (falling) time and a constant voltage, a good quality waveform called “eye open” in an eye mask 60 can be obtained.
FIG. 14 illustrates the example waveform of a poor quality LVDS signal. As the cable length becomes long, the voltage amplitude of the differential signal becomes low and the high frequency component of data due to the frequency characteristic is attenuated, that is, the rising (falling) time of data becomes so long that the waveform invades the eye mask 60, resulting in “eye close”.